1. Field of the Invention
This invention relates to current cells and to a digital-analog converter (DAC) wherein those current cells are used. A DAC outputs electrical currents according to numerical values indicated by digital signals. DACs are employed, for example, in integrated circuits and the like for processing video signals.
2. Description of Related Art
One known current cell used in DACs is the current cell described in U.S. Pat. No. 5,452,014.
As noted in this U.S. patent, a current cell, in principle, comprises one current source and two switches. One switch is a switch that controls current dumping, which is used for making and breaking connections between a constant current source and ground. The other switch is a switch that controls current output, which is used for making and breaking connection between the current source and a current output terminal. When the current dumping switch is closed and the current output switch is open, the current output from the current source is dumped to ground. When, on the other hand, the current dumping switch is open and the current output switch is closed, the current output from the current source is output from the current output terminal to the outside.
Switching transistors can be used for these two switches. In current cells wherein switching transistors are used, there are cases where the transistors are not in a completely on state at exactly the same time. In such cases, the current source cannot output currents at the design values, for which reason the output voltage of the current source rises abnormally. Accordingly, when, subsequent thereto, one of the switching transistors attains a completely on state, a very large current flows to that switching transistor. Parasitic inductance is present in integrated circuit lead wires and in patterns printed on circuit boards. For that reason, large current fluctuations cause cross-shorting and ringing, etc. In DACs mounted in integrated circuits used for video signal processing, cross-shorting and ringing cause deterioration in picture quality.